1. Field of the Invention
This invention relates to an apparatus for controlling engine idle speed for vehicles which have an anti-skid brake system.
2. Description of the Prior Art
When an accelerator pedal is released from being trodden under the gear connection for low speed such as first or second speed, on a low frictional road such as an icy road, the wheel speed or rotational speed V.sub.R of the drive wheel is often remarkably reduced in comparison with the running speed V.sub.F of the vehicle body, by the dragging torque of the engine or engine brake. In other words, the slip ratio ##EQU1## is increased, and there is sometimes the fear that the drive wheel locks.
When a brake pedal is trodden under the above described condition, the rotational speed V.sub.R of the drive wheel is reduced more and more, and so the slip ratio of the drive wheel is increased more and more, although the drive wheel is not locked by the engine brake. When the vehicle is provided with an anti-skid control apparatus, and the drive wheel is subject to the anti-skid control, the brake fluid pressure of the drive wheel cylinder of the wheel is so lowered that the rotational speed of the drive wheel is increased so as to attain the aimed slip ratio (for example, 10 to 30%). The braking frictional coefficient is the maximum within the range of the slip ratio of 10 to 30%. Further, the lateral slip frictional coefficient or so-called "side force" is not zero within the above range of the slip ratio, and accordingly, the braking efficiency is high, and the running stability of the vehicle is good.
However, under the above condition, the brake fluid pressure is reduced to nearly zero, or to zero. Further, the rotational speed of the drive wheel is hard to recover or reaccelerate due to the engine brake. Accordingly, the brake fluid pressure control time is lengthened, and so the braking force becomes insufficient. The braking distance is thereby lengthened. Further, the slip ratio is maintained high during that time. Accordingly, the side-force is small, and the running stability of the vehicle is bad. That is very dangerous.